A body accelerating through a liquid is subject to a very high friction or drag, compared to the same body accelerating through air. Example, water creates roughly 1000 times more drag than air. Contact between the surface of an object moving relative to a surrounding liquid and the liquid creates drag, to a degree depending upon the viscosity of the liquid, because a layer of liquid tends to adhere to the surface of the object and be carried with it. As the speed of the object increases, the slower moving liquid adhering to it causes a boundary zone of turbulent flow. As a result of this, the energy required to move an object through a liquid increases with velocity. In the case of an object moving through water, the energy required to move the object through the water increases cubically with velocity.
The present invention is particularly concerned with devices moving relative to water, and further discussion is with specific reference to water. However, the present invention would also be useful applied to devices moving relative to other liquids.
Water borne craft, in particular, tend to be subject to very high frictional losses because for the craft, to travel at a reasonable speed, requires the impellers to displace water at relatively high rotational velocities. The frictional losses increase the energy input required to achieve a given speed. A further cause of problems is the cavitation effect:—cavitation is the generation of cavities (for example bubbles) in liquids by rapid pressure changes. When an impeller rotates at high speed in water, there is a significant drop in pressure in the water immediately adjacent the impeller and small bubbles form on the surface of the impeller. When these bubbles implode, they produce shock waves in the water which can cause serious erosion damage to the impeller's surface.